Temporary grassland is not a miracle solution for soil quality of arable land: beyond the keywords, the agronomic factors must be considered

Téo Lemaître,Ophélie Sauzet,Pascal Boivin,Xavier Dupla

doi:10.5194/egusphere-egu23-17591

Abstract

&lt;p&gt;Restoring soil quality has become a priority, with special focus on increasing the soil organic carbon (SOC) content which is the major factor of topsoil quality. Therefore, conservation agriculture (CA) techniques receive growing interest. Among the options considered to increase the SOC of cultivated land, the proportion of temporary grassland (TC) in the rotation is often cited.&lt;/p&gt; &lt;p&gt;We monitored the topsoil SOC content of 120 fields from 120 farms in the Swiss Leman Lake region and analysed the relationships between the percentage of TG in the rotation, the other cropping practices, and the observed changes of the SOC content over 10 years.&lt;/p&gt; &lt;p&gt;The cropping practices showing an impact on the annual rate of change (SOC-ARC) in the SOC content were shown to be the Soil Tillage Intensity Rating (STIR), the percentage of bare soil in the year, the cover-crop frequency and diversity, and the farm manure application converted in humified organic matter (Dupla et al., 2022). SOC content was decreasing with increasing STIR and percentage of bare soil, increasing with cover-crop frequency and diversity, and manure application.&lt;/p&gt; &lt;p&gt;The SOC:clay ratio of the monitored fields ranged from 3% to 14% and was on average low compared to the structure vulnerability thresholds quantified in the studies of (Johannes et al., 2017; Prout et al., 2020). The percentage of TG in the rotation ranged from 0% to 70% of the 10-year period, 75 fields had no TG (TG-) and 45 had (TG+). Though the SOC:clay ratios of the TG- fields was significantly smaller than those of the TG fields, the SOC:clay was not increased with the percentage of TG in the TG+. On the whole data set, the percentage of TG showed no correlation with SOC-ARC, but it was positively correlated when considering only the TG+ fields. The impacts of the farm manure, cover-crop diversity, soil cover and STIR on SOC-ARC were increased with increasing TG percentage.&lt;/p&gt; &lt;p&gt;The livestock units per ha were significantly higher for the TG+ fields but were not correlated with the percentage of TG. The farm manure input was not correlated to the TG percentage, but when slurry and digestate were excluded, a positive correlation was observed. The STIR decreased and the percentage of soil cover increased with increasing TG percentage. However, when considering only the main crops in the rotation, the percentage of bare soil and the STIR increased with TG percentage, while the number of species in the cover crops decreased, showing the higher the TG percentage, the more detrimental to the soil were the practices of the farmers in the main crops of the rotation. The number of species in the cover crops was decreasing with increasing TG percentage.&lt;/p&gt; &lt;p&gt;This study revealed that increasing the proportion of temporary grassland is not a guarantee of SOC increase compared to rotations without TG. More detrimental practices may offset the potential benefit of TG, though TG has a synergistic effect with CA practices. Therefore, temporary grasslands cannot be a keyword guaranteeing soil regeneration but can contribute to it provided that the CA agronomic factors are properly applied.&lt;/p&gt; &lt;p&gt;&amp;#160;&lt;/p&gt; &lt;p&gt;Dupla, X., Lema&amp;#238;tre, T., Grand, S., Gondret, K., Charles, R., Verrecchia, E., Boivin, P., 2022. On-Farm Relationships Between Agricultural Practices and Annual Changes in Organic Carbon Content at a Regional Scale. Frontiers in Environmental Science 10, 13.&lt;/p&gt; &lt;p&gt;Johannes, A., Matter, A., Schulin, R., Weisskopf, P., Baveye, P.C., Boivin, P., 2017. Optimal organic carbon values for soil structure quality of arable soils. Does clay content matter? Geoderma 302, 14&amp;#8211;21. https://doi.org/10.1016/j.geoderma.2017.04.021&lt;/p&gt; &lt;p&gt;Prout, J.M., Shepherd, K.D., McGrath, S.P., Kirk, G.J.D., Haefele, S.M., 2020. What is a good level of soil organic matter? An index based on organic carbon to clay ratio. European Journal of Soil Science n/a. https://doi.org/10.1111/ejss.13012&lt;/p&gt;

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